The present invention relates to a method and an arrangement for controlling pump displacement in a work vehicle.
The term “work vehicle” comprises different types of material handling vehicles like construction machines, such as a wheel loader, an articulated hauler, a backhoe loader, a motor grader and an excavator. Further terms frequently used for work vehicles are “earth-moving machinery” and “off-road work machines”. The invention will be described below in a case in which it is applied in a wheel loader. This is to be regarded only as an example of a preferred application. The work vehicles are for example utilized for construction and excavation work, in mines etc.
The work vehicle comprises a powertrain for propelling the vehicle. A power source, preferably an internal combustion engine, and especially a diesel engine, is adapted to provide the power for propelling the vehicle.
The work vehicle further comprises a hydraulic system. The hydraulic system comprises at least one variable displacement pump and at least one actuator operatively driven by hydraulic fluid delivered from said pump. The system may be of load-sensing type, wherein the pump displacement is controlled by a pilot pressure representing a load exerted on the system. The pump is normally operatively driven by the diesel engine.
Said actuator may be a linear actuator in the form of a hydraulic cylinder. A wheel loader comprises several such hydraulic cylinders in order to perform certain functions. A wheel loader is frame-steered and a first pair of hydraulic cylinders is arranged for turning the wheel loader. Further, there are hydraulic cylinders provided for lifting a load arm unit and tilting an implement, for example a bucket, arranged on the load arm unit.
A load sensing hydraulic system is characterized by that the operating condition of the load is sensed and that the output pressure of the pump is controlled so that it exceeds the load pressure existing in the hydraulic actuator by a predetermined differential.
In order for the work vehicle to function well, the engine, transmission and hydraulic system must be balanced with regard to available power and output power. It is difficult to find an engine that exactly manages the desired power outputs at different engine speeds. The problem with different output power demand is particularly pronounced at low engine speeds. If the driver utilizes the power from the engine at low engine speeds to drive the vehicle's half shafts at the same time as the hydraulic system is activated, then there is a risk that the engine will cut out or that the engine will “stick”, that is it will not be able to increase the engine speed when the driver depresses the accelerator pedal. The driver can, of course, adjust the power consumption via various controls, when he senses a loss of engine speed, but this can be problematical, particularly when the engine suddenly cuts out. Further, even skilled drivers overcompensate and therefore unnecessarily reduce the amount of hydraulic work the hydraulic system is truly capable of performing. As a result, machine productivity is reduced.
It is desirable to achieve a method for controlling pump displacement in a work vehicle with a load-sensing hydraulic system that creates conditions for limiting the hydraulic power in order to relieve the load on the power source when necessary. The invention is especially directed to a work vehicle with an internal combustion engine as power source and the method particularly aims for relieving engine load, especially when there is a risk for stalling the engine.
A method according to an aspect of the present invention comprises the steps of                detecting an operational condition of a powertrain which is adapted to propel the vehicle, wherein a power source in the powertrain is adapted to operatively drive at least one variable displacement pump, wherein the pump is adapted to operatively drive at least one hydraulic actuator via hydraulic fluid for moving a work implement and/or steering the vehicle,        comparing the detected operational condition with a predetermined critical condition,        limiting a maximum available displacement of the pump and thereby establishing an available pump displacement range if the magnitude of the detected operational condition is within the predetermined critical condition,        detecting a hydraulic load associated to the actuator, and        adjusting the pump displacement of the pump in response to the detected hydraulic load within the established available pump displacement range.        
In fact, the maximum pump capacity is decreased by means of the step “limiting a maximum available displacement of the pump and thereby establishing an available pump displacement range”. Thus, when the critical condition is reached, the maximum capacity of the pump is decreased, wherein the pump will function as a smaller pump than it is in fact. The pump will always/continuously function as a normal load-sensing pump up to the established maximum available displacement. Preferably, the maximum available pump displacement is controlled proportionally with regard to the magnitude of the detected operational condition within the critical condition range.
The powertrain is adapted to propel the vehicle via ground engaging members (wheels or crawlers). The powertrain comprises the power source and a system for transmitting power from the power source to the ground engaging members. According to one preferred example, the powertrain is of a mechanical type and preferably comprises from the power source to the ground engaging members the following: a clutch and/or a torque converter, a transmission, a cardan shaft, a differential gear and transverse half shafts.
The power source (prime mover) is adapted to provide a motive power for propelling the vehicle and to operatively drive the variable displacement pump. The power source is preferably an internal combustion engine, especially a diesel engine.
The predetermined critical condition is preferably formed by a condition range and is indicative of a risk for the power source being overloaded, such as engine lugging/engine shutting off. Preferably, an operational condition of the power source itself is detected.
The actuator is adapted to perform a work function (moving a work implement, such as a bucket or forks) or steer the work vehicle. The actuator is preferably formed by a hydraulic cylinder. The actuator is controlled by manual operation of a control element (lever or joystick).
The hydraulic load associated to the actuator is indicative of an external load exerted on the actuator from a steering operation or from operation of the implement. The load is preferably detected by sensing a hydraulic pressure in a hydraulic system comprising the pump and actuator(s). The displacement of the pump is preferably automatically adjusted within the established pump displacement range in response to the sensed hydraulic pressure. Thus, the hydraulic system is preferably of a load sensing type.
According to a preferred embodiment, the method comprises the steps of determining the maximum available pump displacement on the basis of the magnitude of the detected operational condition. Preferably, the maximum available pump displacement is limited to a larger extent upon a smaller magnitude of the detected operational condition.
According to a further preferred embodiment, the method comprises the steps of continuously variably controlling the magnitude of the limitation of the maximum available pump displacement on the basis of the magnitude of the detected operational condition. Thus, the maximum available pump displacement could be fast and accurately controlled in response to a change in the operational condition.
According to a further preferred embodiment, the method comprises the steps of detecting a hydraulic pressure associated to the actuator, comparing the detected hydraulic pressure with a predetermined limit value and only limiting the maximum available pump displacement if the detected hydraulic pressure is above the predetermined limit value. Thus, there is no need for limiting the maximum available pump displacement if the detected hydraulic pressure is below the predetermined limit value.
According to a further preferred embodiment, the method comprises the steps of detecting a torque or output power of the power source and limiting the maximum available pump displacement if the magnitude of the detected torque or output power is below a predetermined torque or output power value. Thus, there is no need for limiting the maximum available pump displacement if the detected torque or output power is above the predetermined limit value.
It is desirable to achieve an arrangement for controlling pump displacement in a work vehicle with a load-sensing hydraulic system that creates conditions for limiting the hydraulic power to relieve the load on the power source when necessary. An aspect of the invention is especially directed to a work vehicle with an internal combustion engine as power source and the arrangement particularly aims for relieving engine load, especially when there is a risk for stalling the engine.
An arrangement according to an aspect of the invention comprises                at least one variable displacement pump operatively driven by a power source,        at least one actuator operatively driven by hydraulic fluid delivered from said pump for moving a work implement and/or steering the vehicle,        means for detecting an operational condition of a powertrain, wherein the powertrain comprises the power source and is adapted to propel the vehicle,        means for comparing the detected operational condition value with a predetermined critical condition,        means for limiting a maximum available displacement of the pump and thereby establishing an available pump displacement range if the magnitude of the detected operational condition is within the predetermined critical condition,        load sensing means for detecting a hydraulic load associated to the actuator, and        means for adjusting the pump displacement in response to the detected hydraulic load within the established available pump displacement range.        
Further preferred embodiments and advantages will be apparent from the following description and drawings.